Making the assumption that k-1 is far greater than k+2 is an assumption that is true for many enzymes. Forming an enzyme-substrate complex is unlikely to be completely correct, resulting in the complex dissociating. However, this isn’t true for all reactions – enzymes with a very high Kd for their substrate would have less of a disparity between the two values.
However, making the assumption that k-1 » k+2 makes calculating kinetic values far easier. This is because, during the initial stages of the reaction, it is possible to assume that no product has been made, and therefore simple unimolecular reversible reaction equations can be used. As product begins to form, more factors need to be taken into account, making the kinetic equations more complex.